A water softening apparatus consists of a tank containing water softening material which receives hard water from a supply line and which softens the water passing through the tank before being supplied through a service outlet. The ion exchange capability of the water softening material becomes depleted after a predetermined amount of hard water has passed through the bed. Regeneration consists of reactivating the bed with materials, such as brine. Electrically conductive probes are set in the water softening tank to determine when regeneration is needed. The principle of comparative sensing is based on the fact that water and water softening material together have varying conductivity from a charged (sodium or soft) state to a depleted (calcium or hard) state.
In Canadian patent Nos. 1,136,088 and 1,167,547, assigned to Culligan International Company, two pairs of electrodes are facing into the water softener bed and water. As the bed depletes, the top set of electrodes changes in conductivity and when the difference between the two sets of electrodes reaches a predetermined difference, means are latched to initiate a regeneration at a preset time. However, this system does not consider the fact that incoming water may vary from the water in the tank, in temperature as well as in total dissolved solids (TDS) and that this may affect the conductivity of the top set of probes, irrespective of the state of the bed of ion exchange resin and the hardness of the water at that point. In effect, the electrodes could be fooled into reading a depleted situation and causing a premature regeneration as a result of changes in temperature or in total dissolved solids.
In U.S. Pat. No. 3,768,649, assigned to Andrew Fleckenstein, one set of probes faces into a resin bed and water and a second set faces into a substantially enclosed chamber containing depleted resin and hard water. As the resin bed in the tank depletes, the conductivity of the first set of probes approaches that of the probes located in the standard. When a predetermined value is reached, a regeneration cycle is initiated at a preset time. However, adverse water conditions, particularly iron fouled water, tend to contaminate the electrodes and resin in the enclosed standard and plug the tiny purge hole which is provided to permit the flow of hard water through the standard. When this happens, the conductivity readings of the standard do not represent true values and the unit malfunctions, severely limiting the application of the sensing system in iron fouled waters.